Public concern about air quality and the federal air laws have created a large demand for low emission vehicles. One form of a low emission vehicle is an electric vehicle, and the first electric powered vehicles employed transmissions that received power from a source of stored electrical energy, such as a battery. Energy from the battery powered an electric motor which, in turn, drove the wheels of the vehicle through a transmission. When the charge on the battery was expended, there was no more energy available to move the vehicle, and the vehicle could not move on its own until the battery was recharged. In order to extend the range of such vehicles, an engine and a generator were incorporated to convert a fossil fuel into electrical energy. Typically, the engine would drive the generator, and the resulting electrical energy was directed either to the battery, so that it might be recharged, or to one or more electrical motors that could assist in driving the vehicle wheels. This type system is designated as a series hybrid propulsion system.
In short, a series hybrid system is a system in which energy follows a path from an engine to a battery and then to an electrical motor which applies power to rotate the wheels. There is no mechanical path between the engine and the wheels in a series hybrid propulsion system.
Series hybrid propulsion systems for vehicles are generally designed with a low power engine for minimum emissions and high fuel economy. Such systems are inefficient when the vehicle requires high average power output or operates at continuous constant speeds. Moreover, high efficiency is not available when the vehicle is required to climb steep grades or when the vehicle must sustain high average cruising speeds. It is also recognized that the series hybrid transmission requires sizable motor/generators and must be available in a wide variety of motor sizes that are individually acceptable for specific vehicle weights and loads. The challenge is to provide a power system that will operate at high efficiencies over a wide variety of operating conditions.
Desirable electric variable transmissions should leverage the benefits of a series hybrid transmission for desirable low average power duty cycles--i.e.: low speed start/stop duty cycles--and a parallel hybrid for high average power, high speed duty cycles.